doc: fix typo in streams.md

Slight grammar change to the readable event:`error`

PR-URL: https://github.com/nodejs/node/pull/12924
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/stream.md

@@ -684,7 +684,7 @@ added: v0.9.4
 * {Error}
 
 The `'error'` event may be emitted by a Readable implementation at any time.
-Typically, this may occur if the underlying stream in unable to generate data
+Typically, this may occur if the underlying stream is unable to generate data
 due to an underlying internal failure, or when a stream implementation attempts
 to push an invalid chunk of data.